Many different devices and methods have been demonstrated to mode-lock a laser to generate an optical pulse train from the laser. These devices and methods include both active mode-locking and passive mode-locking approaches.
Active mode-locking uses an amplitude modulator inside the cavity to modulate a loss at a rate equal to the laser round-trip frequency, resulting in a pulse train. Though relatively simple to design and implement, active mode-locking has several limitations. One of the major disadvantages of active mode-locking is the difficulty of scaling down the pulse width; the pulse width generally ranges from several picoseconds to tens of picoseconds for active mode-locking.
On the other hand, in passive mode-locking, a device with intensity-dependent loss (generally referred to as a “saturable absorber”) is placed in the cavity. If the loss decreases with increasing intensity, pulse formation is favored. Passive mode-locking generates much shorter pulses than does active mode-locking because the pulse inside the laser self-modulates itself more rapidly than it does in any active modulation. Depending on how fast the saturable absorber recovers to its default state after being saturated by a single pulse with a given pulse width, it is classified either as a slow saturable absorber (SSA) or as a fast saturable absorber (FSA). The slow saturable absorber has a recovery time longer than the pulse width of the saturating pulse, while the fast saturable absorber has a shorter recovery time.
For femtosecond pulse generation, the slow saturable absorber is generally implemented in the form of a compact semiconductor saturable absorber mirror with widely adjustable parameters for mode-locking various kinds of lasers. In passive mode-locking using a slow saturable absorber, the width of the generated pulses is generally limited from a picosecond to several picoseconds. In addition, when the absorber parameters are inappropriate, the laser can operate in Q-switched mode-locking where the pulse train is modulated with a frequency much lower than the cavity round-trip time.
The fast saturable absorber is generally implemented in the form of an artificial saturable absorber using a nonlinear phase shift. A femtosecond pulse (i.e., a pulse having a width of 1-500 femtoseconds; e.g., a pulse in a fiber with a pulse width in the range from 50 to 200 femtoseconds) is generally generated based on the fast-saturable-absorber mechanism by Kerr-lens mode-locking, additive-pulse mode-locking, and nonlinear-polarization-rotation mode-locking.
Femtosecond sources with multi-gigahertz repetition rates at optical communications wavelengths are important building blocks for numerous applications, such as femtosecond laser frequency comb generation for frequency metrology, optical arbitrary waveform generation, high-speed optical sampling, and the calibration of astrophysical spectrographs. Currently, only a few approaches meet the stringent requirements in terms of pulse duration, repetition rate, operating wavelength, and noise performance simultaneously. These current approaches, however, are bulky, expensive and of limited robustness because they employ external Fabry-Perot filters locked to the mode comb for multiplying the repetition rate to the multi-gigahertz range, either inside or outside of the low-fundamental-repetition-rate laser cavities.
With the constraints of achieving femtosecond pulse duration and low timing jitter, passive mode-locking provides a path to reach a multi-gigahertz fundamental repetition rate. Record-high repetition rates of a few hundred MHz have been reported in previous attempts using polarization additive pulse mode-locking (P-APM). Additionally, passively mode-locked fiber lasers using saturable Bragg reflectors (SBRs) with repetition rates up to 2 GHz have been reported. These attempts, however, failed to produce femtosecond pulses, which are important for low-jitter and for frequency-metrology applications.